1. Field of the Invention:
The present invention relates to a thin film deposition apparatus, and, more particularly, to a thin film deposition apparatus for forming a thin film by the cluster ion beam deposition method.
2. Description of the Prior Art:
A thin film deposition method employing the cluster ion beam deposition method has been proposed in, for example, the specification of Japanese Patent Publication No. 9592/1979. In such a method, vapors of a substance to be deposited on a substrate are ejected within a vacuum chamber as clusters in each of which a large number of atoms in the stream are loosely combined. These clusters are bathed with a shower of electrons so as to generate cluster ions in each of which an atom is ionized, and these cluster ions are caused to accelerate and collide with the substrate, by means of which a thin film is formed on the substrate.
FIG. 1 shows a known thin film deposition apparatus which is used to carry out such a thin film deposition method. In the apparatus, a vacuum chamber 1 in which a predetermined degree of vacuum is maintained is provided with an evacuation passage 2 through which the vacuum chamber is evacuated. The evacuation passage 2 is connected to an evacuation device (not shown), and is opened and closed by means of a vacuum valve 3. A sealed crucible 4 in the vacuum chamber 1 has a nozzle 4a having a diameter of 1 mm to 2 mm. It contains a substance which is to be evaporated so as to be deposited on the substrate, e.g., zinc (Zn) 5. A bombarding filament 6 for irradiating thermoelectrons on the crucible 4 so as to heat it is disposed in such a manner as to surround the sealed crucible 4. A heat shielding plate 7 which acts as a shield against the radiant heat from the filament 6 is provided around the filament 6. The crucible 4, the bombarding filament 6, and the heat shielding plate 7 form in combination a vapor generating source 8 for generating clusters which is achieved by ejecting the vapors of a substance to be deposited on the substrate within the vacuum chamber 1.
An ionizing filament 9 located above the vapor generating source 8 is heated by a power source (not shown), whereupon it emits thermoelectrons 13 to ionize the clusters formed of the vapors of the substance to be deposited on the substrate. An electron extracting elctrode 10 is adapted to accelerate the thermoelectrons 13 emitted from the ionizing filament 9. The radiant heat from the ionizing filament 9 is intercepted by a heat shielding plate 11 provided adjacent to the heat shielding plate 7 through an insulated supporting member 23. The ionizing filament 9, the electron extracting electrode 10, and the heat shielding plate 11 form in combination an ionizing means 12 for ionizing the clusters emitted from the vapor generating source 8. The inner side of the ionizing filament 9 forms an ionizing section 12a.
An accelerating electrode 14 serves to accelerate the ionized cluster ions 16, which, together with non-ionized neutral clusters 15, causes them to collide against a substrate 18 so as to deposit a thin film on the substrate 18. A potential can be generated between the accelerating electrode 14 and the electron extracting electrodes 10. In addition, a cluster beam 17 consists of the cluster ions 16 and the neutral clusters 15. The crucible 4 is supported by a support 20, and the support 20 and the heat shielding plate 7 are supported by the vacuum chamber 1 through an insulated supporting member 19. The accelerating electrode 14 is disposed adjacent to the ionizing means with an insulated supporting member 24 interposed therebetween. The substrate 18 is supported by a substrate holder 22, and the substrate holder 22 is in turn supported by the vacuum chamber 1 through an insulated supporting member 21.
With the thus-arranged thin film deposition apparatus, a thin film of, for example, zinc is deposited on the substrate 18 as follows: first, the zinc is charged in the crucible 4 as the substance to be deposited on the substrate, then the vacuum chamber 1 is evacuated by the evacuating device until the interior of the vacuum chamber 1 exhibits a degree of vacuum of about 10.sup.-6 mm Hg. Next, the bombarding filament 6 is energized so as to be heated. The temperature of the zinc 5 contained in the crucible 4 rises due to the radiant heat from the bombarding filament 6 or the collision of the thermoelectrons emitted from the filament 6 with the crucible 4, i.e., by electron impact, to a point (500.degree. C.) at which the vapor pressure thereof is about 0.1 to 10 mm Hg. As a result, zinc vapors are ejected from the nozzle 4a, adiabatically expand due to the difference in the pressures in the crucible 4 and the vacuum chamber 1, and thereby form clusters in each of which a large number of atoms are loosely combined.
The thus-formed cluster beam 17 is caused to collide with the thermoelectrons extracted from the ionizing filament 9 by the electron extracting electrode 10. In consequence, one of the atoms in each of the clusters which constitute part of the cluster beam 17 is ionized, thereby forming a cluster ion 16. The cluster ions 16 are accelerated by an appropriate degree by means of the electric field generated between the accelerating electrode 14 and the electron extracting electrode 10. The accelerated cluster ions 16 collide with the substrate 18, as the neutral clusters 15 collide with the substrate 18 by virtue of the kinetic energy with which they are imparted as they are ejected from the crucible 4, whereupon a thin zinc film is deposited on the substrate 18. FIG. 2 is a perspective view showing the state in which the cluster beam 17 collides with the substrate 18.
FIG. 3 shows the electric potentials in the components located in the vicinity of the ionizing means 12, as well as the potential distribution in the space located in the vicinity of the ionizing means 12 in the known thin film deposition apparatus. The broken lines in FIG. 3 denote potential contours representing 0.8 kV, 1.0 kV, . . . , 4.0 kV, 4.7 kV, and the numbers in parentheses designate the electric potentials in the components of this thin film deposition apparatus. These contours are obtained by calculations. The hatched area represents an area surrounded by the contour (4.7 kV) of the same potential as that of the ionizing filament 9 and in which area thermoelectrons can exist.
In the above-described known thin film deposition apparatus, the thermoelectrons 13 exist in the vicinity of the center of the ionizing portion 12a as well, as can be seen from FIG. 3. Therefore, the cluster ions 16 become concentrated on the center of the substrate 18 when they reach the substrate. This results in a non-uniform ion distribution, which leads to formation of a non-uniform thin film in terms of film thickness and film quality.